Ion Nitriding of Small Holes and Narrow Slits

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In the process of ion nitriding, the small holes and narrow grooves on the parts often fail to be nitridated (or the shallow hardness of the infiltrated layer is low), glow concentration and arc discharge are three problems. Through reasonable furnace loading and process parameter control, ion nitriding of small holes and narrow slots can be realized.

According to the principle of glow discharge, the four parts of Aston dark zone, cathode glow zone, cathode dark zone and negative glow zone are necessary to maintain the gas glow discharge. The total thickness of the above-mentioned area is called the cathode discharge length. This parameter is only more than the thickness of the negative glow zone than the cathode drop zone dk. When the cathode drop zone dk is larger than the radius of the small hole or the half-width of the narrow groove, the internal glow cannot be maintained and extinguished, so that the inside of the hole and groove cannot be infiltrated with nitrogen. When the inner diameter of the hole or the half width of the groove is larger than dk and smaller than the cathode discharge length, the glow in the hole and groove overlaps each other to increase the light intensity. The electron emission caused by the photoelectric effect causes more electrons to be generated on the inner wall of the hole and groove, thus exhibiting a higher current density and an unusually bright glow concentration phenomenon. The concentrated glow makes the small holes and narrow grooves quickly overheat, and if thermionic emission is caused, arcing will occur. When the cathode discharge length is less than one-half of the aperture or slot width, the concentration of glow can be avoided, and ion nitriding can proceed smoothly in those places. Therefore, an important condition for achieving ion nitriding of small holes and narrow grooves is to make the cathode discharge length less than half of the hole diameter and groove width.

The length of the cathode discharge is mainly affected by the working pressure. In addition, the working atmosphere, part temperature and current density also have varying degrees of influence. Increasing the working pressure can reduce the length of the cathode discharge. When the working pressure is 8-10 Torr, the discharge length of the cathode is only about 1mm, so that the ion nitriding can be carried out on a relatively narrow hole. Secondly, when using a mixture of nitrogen and hydrogen as the source, the more nitrogen content, the shorter the cathode discharge length.When the discharge length of the cathode is required to be particularly small, argon gas can also be appropriately added to enable the glow to enter the particularly small holes. As the temperature of the parts increases and the discharge length of the cathode increases, the air pressure should be increased at any time to control the discharge length to be small enough to avoid the glow concentration in the hole until it rises to the nitriding temperature.

In the ion nitriding process, nitriding of small holes and grooves can be achieved by controlling some process parameters such as reasonable air pressure, working atmosphere, current density and so on.

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